Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. As the device density on IC chips has increased, the size of the structures making up the devices has approached the wave length (around 0.5 .mu.m) of the light used in optical photolithography processes. This correspondence in size of the structure and wave length of the incident light, together with defraction, interference and/or light divergence phenomenon, can adversely affect the resolution of optical photolithography to an extent that future density increases in IC devices may be difficult to achieve absent the development of alternative lithographic technologies.
In part as a result of these limitations of optical photolithography, X-ray lithography was developed. Due to the shorter wave length of the soft X-rays used in X-ray lithography, generally about 0.1 to 1.0 nanometers, resolution is improved significantly. With increased resolution it becomes possible to increase the device density on IC chips and/or increase chip yield. X-ray lithography has yet to be widely adopted by the semiconductor industry due, in part, to the relatively high cost of the synchrotron and other equipment used in X-ray lithography.
Phase-shift lithography was developed to enhance the range of conventional optical photolithography. Phase-shift lithography is based on opposite phase destructive interference of the waves of incident light. By shifting the phase of one region of incident light waves 180.degree. relative to an adjacent region of incident light waves, a sharply defined dark zone is created beneath the phase-shift mask due to destructive interference of the waves. This zone defines the interface between light and dark regions, and hence defines the boundary between exposed and unexposed portions of the resist layer underlying the phase-shift mask.
Several different phase-shift lithography techniques have been developed. One of the earliest techniques developed, as reported by Levenson et al. in the article "Improved Resolution in Photolithography with a Phase Shifting Mask," IEEE Transactions on Electron Devices, Vol. ED-29, Nov. 12, December, 1982, pages 18-36, involves the use of a periodic pattern arrangement in the transmission mask. Although the Levenson technique provides very sharp image contrast, it tends to be relatively difficult to fabricate phase shift masks for the technique. Such fabrication difficulties arise from the requirement that mask openings be arranged in a periodic fashion that typically does not conform exactly to the desired design layout needed for printing.
Another phase shift lithography technique was developed, in part, to avoid the relatively complex mask fabrication requirements of the Levenson technique. This alternative phase-shift lithography process, known as self-aligned phase-shift lithography or rim-type phase-shift lithography, has been reported by Todokoro et al. in the article "Self-aligned Phase Shifting Mask for Contact Hole Fabrication," Microelectronic Engineering, Nov. 13, 1991, pages 131-134, and by Ishiwata et al. in the article "Fabrication of Phase-Shifting Mask," Proceedings of the SPIE, Vol. 1463, pages 423-33, 1991.
As illustrated in FIG. 1, rim-type phase-shift lithography involves the use of a mask 10 comprising a substrate 12 made from a material through which incident electromagnetic radiation 14 used in the phase-shift lithography process will propagate. Substrate 12 has a plurality of recessed portions 16 separated by mesas 17. Sidewalls 18 define the boundary between recessed portions 16 and mesas 17. Portions of the top surfaces of mesas 17 are covered with a blocking layer 20, e.g., a layer of chromium, through which electromagnetic radiation 14 cannot be transmitted. Blocking layer 20 is formed so that edges 22 thereof are "pulled back" somewhat (i.e., are moved horizontally inwardly) relative to adjacent sidewalls 18, thereby exposing portions 24 of the top surface of substrate 12.
The height of sidewall 18, i.e., the distance between surface portion 24 and the base of recess 16, is selected so that electromagnetic radiation 14 impinging the back side of substrate 12 and propagating through portion 26 (FIG. 1) of substrate 12 exits surface portion 24 180.degree. out of phase with electromagnetic radiation impinging the back side of the substrate and propagating through portion 28 (FIG. 1) of the substrate and exiting the bottom of recessed portion 16 adjacent sidewall 18. This phase shift occurs because substrate 12 modifies the phase of electromagnetic radiation transmitted therethrough an amount that varies as a function of the refractive index and the thickness of the substrate through which the electromagnetic radiation is transmitted. Thus, knowing the extent to which substrate 12 modifies the phase of electromagnetic radiation 14 transmitted therethrough, recessed portions 16 are formed to a depth sufficient to cause the 180.degree. phase shift of electromagnetic radiation 14 described above. Due to destructive interference between electromagnetic radiation 14 transmitted through substrate portions 26 with radiation transmitted through substrate portions 28, a nearly vertical interface in the intensity of radiation transmitted through mask 10 is formed extending substantially along planar extensions of sidewalls 18. The graph above mask 10 in FIG. 1 illustrates the intensity of electromagnetic radiation 14 transmitted through mask 10, with nearly vertical lines 30 representing the sharp light/dark interface extending substantially along planar extensions of sidewalls 18.
It is important that the extent to which blocking layer 20 is pulled back relative to sidewall 18 be precisely controlled. As illustrated in FIG. 2a, if sufficient pull back of blocking layer 20 is not achieved, and hence only a relatively small surface portion 24 is exposed, substantially no destructive interference of the transmitted radiation occurs. As such, the intensity of radiation intercepting underlying resist layer 40 drops off gradually relative to planar extensions of sidewalls 18. Consequently, the patterns activated on resist layer 40 are larger than is desired. By contrast, as illustrated in FIG. 2b, if blocking layer 20 is pulled back too much relative to sidewall 18, thereby creating relatively large surface portions 24, some of the radiation propagating through the mesa 17 will not destructively interfere with radiation transmitted through adjacent recessed portions 16. Consequently, two regions 42 (FIG. 2b) of relatively high intensity electromagnetic radiation will occur at resist layer 40. These high intensity regions 42 can result in the activation of portions of resist layer 40 that are not intended to be activated, with the result that undesired structure may be formed in the underlying wafer during subsequent process steps.
Unfortunately, implementation of rim-type phase-shift lithography has been limited due to the absence of processes for forming phase-shift masks that permit the extent of pull back of blocking layer 20 to be controlled to the extent required to avoid the above-described undesirable effects illustrated in FIG. 2a and 2b. For example, in processes for forming rim-type phase-shift masks of the type disclosed by Ishiwata et al., as referenced above, blocking layer 20 is pulled back relative to sidewalls 18 by horizontal etching, which process tends to be difficult to control. More specifically, referring to FIGS. 3a-3d, preliminary steps in the Ishiwata et al. process produce the structure illustrated in FIG. 3a. As described above in connection with the discussion of FIG. 1, the structure illustrated in FIG. 3a comprises a quartz substrate 12 in which a plurality of recessed portions 16 are formed, each defined by sidewalls 18. The top surface of substrate 12 is covered with a chromium blocking layer 20. The process step illustrated in FIG. 3a involves covering the recessed portion 16 and blocking layer 20 with a layer of positive resist 50. Then, the backside of substrate 12 is exposed to light, indicated by arrows 52, which propagates through quartz substrate 12 and into resist layer 50, except where transmission of such light into the resist layers is blocked by chromium layer 20.
Next, portions of resist layer 50 activated by light 52 are developed and removed, as illustrated in FIG. 3b. As a result of this backside exposure and subsequent removal of activated portions of the resist layer, unactivated portions of the resist layer remain on top of chromium blocking layer 20. These unactivated portions have substantially vertical sidewalls that are substantially coplanar with the vertical sidewalls of chromium blocking layer 20 and sidewalls 18 of recessed portions 16.
Referring to FIG. 3c, the mask is then subjected to an isotropic (non-directional) plasma etching process for a period of time sufficient to remove those portions of chromium blocking layer 20 adjacent the plane of sidewalls 18 so as to expose portions 24 of the top surface of substrate 12. Because blocking layer 20 is covered with resist layer 50, the blocking layer is etched almost exclusively in a horizontal direction. Hence, the process illustrated in FIGS. 3a-3d may be characterized as a horizontal etching process. The horizontal extent to which layer 20 is removed is indicated in FIGS. 3c and 3d as pull back distance "Y. "
Finally, as illustrated in FIG. 3d, the remaining portions of resist layer 50 are removed.
With the Ishiwata et al. process described above, it tends to be difficult to control the extent of pull back of blocking layer 20 relative to sidewalls 18 because portions of layer 20 are removed substantially linearly with time until little or no blocking layer remains. Consequently, if the plasma etching of chrome blocking layer 20 is conducted for slightly more or less time than is required, the pull back distance Y of layer 20 can vary to an extent sufficient to result in the creation of a mask that produces the undesirable effects illustrated in FIGS. 2a and 2b and described above. Additionally, variations in distribution of the plasma of the etching process can also produce unacceptably large variations in the extent of pull back of chrome blocking layer 20 across a mask surface.
Therefore, a need exists for a method of forming a rim-type phase-shift mask of the type illustrated in FIGS. 1 and 3d that is sufficiently controllable such that only an acceptably small variation in the extent of pull back of blocking layer 20 occurs across a mask surface.